1. Field of the Invention
The present invention relates to a method used in a communication device in a wireless communication system, and more particularly, to a method for buffer status report in dual connectivity.
2. Description of the Prior Art
3GPP in Release 12 proposes dual connectivity for increasing user's throughput. Dual connectivity to at least two cells may be served by different evolved NodeBs (eNB), linked with non-ideal backhaul, e.g., there may be an eNB in charge of a cluster of cells. Therefore, a user equipment (UE) may be served by multiple eNBs when it is in dual connectivity mode.
Under framework of the dual connectivity, traffic streams may be split over more than one eNBs depending on QoS requirements of each traffic type, loading situation, channel condition, and the combination thereof. In detail, in Dual Connectivity, a radio bearer, for which its radio protocols are located in both a master eNB (hereafter called MeNB) and a secondary eNB (SeNB) to use both the MeNB and the SeNB resources, is defined as a split radio bearer. Specifically, for a split radio bearer, the MeNB is U-plane connected to the S-GW via S1-U and the (shared) PDCP routs PDCP PDUs to the RLC entity (namely M-RLC) related to MeNB and the RLC entity related to SeNB (namely S-RLC) for data transmission. Please refer to FIG. 1 for uplink transmission of split radio bearers.
FIG. 1 illustrates protocol structure of a UE connecting to a MeNB and a SeNB. As shown in FIG. 1, uplink data of the radio bearer RB1 starts to be split between packet data convergence protocol (PDCP) entity and radio link control (RLC) entity of the UE. For the radio bearer RB1, there is a shared PDCP entity and two RLC entities M-RLC and S-RLC and two MAC entities M-MAC and S-MAC at the UE for uplink data transmission to the MeNB and SeNB. In detail, the UE transmits uplink data of the radio bearer RB1 from the PDCP entity to the RLC entity M-RLC and the MAC entity M-MAC toward the MeNB, and simultaneously to the RLC entity S-RLC and the MAC entity S-MAC toward the SeNB. As can be seen, uplink data carried by the radio bearer RB1 is split over the MeNB and SeNB. It is noted that this kind of radio bearer is called a split radio bearer in this article. On the other hand, a radio bearer on which the uplink data is not split over multiple eNBs is called a non-split radio bearer.
For uplink resource allocation, a buffer status report (BSR) report is transmitted from a UE to an eNB, so that the eNB can allocates accurate amount of uplink resource to the UE for uplink data transmission. In detail, a buffer size indicated in the BSR report is calculated based on total data amount in PDCP buffer and RLC buffer of logic channels (LCs) of a logical channel group (LCG). In addition, the reported buffer size is mapping to an index based on a buffer size table predetermined in the eNB and the UE. The BSR report can be a short BSR report or a long BSR report, and is reported with MAC sub-header and MAC CE. A format of MAC sub-header and MAC CE for BSR report should be well known in the art, so it is omitted herein.
The applicant notices a problem associated to the BSR report based on the current specification. Please refer to FIG. 2, which illustrates non-split radio bearers according to the prior art. As shown in FIG. 2, it is one to one mapping between radio bearer and logical channel in a UE, i.e. radio bearers RB1-RBn map to logical channels LC1-LCn respectively. However, in dual connectivity, it is never concerned that how the UE reports the BSR for a split radio bearer. Please refer back to FIG. 1, the split radio bearer RB1 is mapped to two logical channels M-LC1 and S-LC1 which are maintained in two MAC entities M-MAC and S-MAC respectively. Accordingly, both MAC entities M-MAC and S-MAC will send BSRs associated with the corresponding logic channel M-LC1/S-LC1 of the split radio bearer to the MeNB and SeNB, respectively. Since there is only a shared PDCP entity for the split radio bearer, based on the abovementioned manner, the PDCP buffer size of the split radio bearer would unavoidable be included in both the BSRs associated with the corresponding logic channel M-LC1/S-LC1. Consequently, the reported buffer size may exceed the real uplink data amount that the UE needs to transmit. Both MeNB and SeNB may allocate the uplink resource according to the received BSRs and the uplink resource would be wasted accordingly because the PDCP buffer size is doubly counted.